In the field of display devices such as a liquid crystal display, there is a demand for improved front side brightness. Therefore, in a backlight for use in such a display, a lens sheet used to collect a light beam from a surface light source to the front surface and improve the front side brightness is provided. A prism sheet as disclosed by JP 3262230 B is generally used as such a lens sheet.
With reference to FIGS. 18 and 19, the conventional prism sheet 100 has prism liners PL arranged (hereinafter simply as “prisms”) on its surface. The refractive index of the prism sheet 100 is about 1.5 to 1.6. Diffused light R100 from a surface light source is refracted at the plane PL of a prism and emitted as it is deflected to the front surface direction. In this way, the prism sheet 100 improves the front side brightness of the display by collecting the diffused light to the front surface.
However, the prism sheet 100 can improve the front side brightness while it also raises the brightness in the front side oblique direction. The solid line in FIG. 20 shows the angular distribution of brightness for the vertical viewing angle of the prism sheet 100 having the prisms PL arranged in the vertical direction (that corresponds to the vertical direction of the display screen). With reference to FIG. 20, the relative brightness has a first peak at a viewing angle in the range of ±30° and a second peak (so-called side lobe) at a viewing angle of +50° or more and at a viewing angle of −50° or less in the front surface oblique direction. Unlike a natural angular distribution of brightness in which the brightness is peaked at a viewing angle of 0° and gradually lowered as the viewing angle widens, the angular distribution of brightness denoted by the solid line in FIG. 20 is unnatural. The user might have unnatural impressions because of the side lobes in some cases as he/she watches the display. Therefore, it is preferable that light that forms side lobes (hereinafter referred to as “side lobe light”) can be restrained from being emitted so that the side lobes can be restrained.
Light for the amount of side lobes cannot be collected to the front surface and therefore the improvement of the surface side brightness using a single prism sheet has its limit. In order to further improve the front side brightness, two prism sheets must be placed on each other and provided on the surface light source, which complicates the manufacturing process.
Furthermore, a prism has a triangular cross section and therefore can easily be damaged during its manufacture, transport and installment to a backlight, particularly at its apex. Such a defect is likely to result in a bright point or a dark point on the display. In order to prevent such defects, the prism sheet 100 before being installed into a display device must be provided with a protection film.